Exemplary embodiments relate to memory, and more specifically, to noncontact writing using spin-currents.
Magnetoresistive random access memory is a non-volatile computer memory (NVRAM) technology. Unlike conventional RAM chip technologies, in MRAM data is not stored as electric charge or current flows, but by magnetic storage elements. The elements are formed from two ferromagnetic plates, each of which can hold a magnetic moment, separated by a thin insulating layer. One of the two plates is a reference magnet set to a particular polarity; the other's magnetic moment can be stabilized in different directions, each direction signifying a different state, serving a storage (memory) function, and is termed the “free magnet” or “free-layer”. This configuration is known as a spin valve if the two magnetic layers are separated by a nonmagnetic metal spacer, and is known as a magnetic tunnel junction if the separation layer is an insulating tunnel barrier. This is the basic building block for a MRAM bit. A memory device is built from a grid of such “cells”.
One method of reading is accomplished by measuring the electrical resistance of the cell. A particular cell is (typically) selected by powering an associated transistor which switches current from a supply line through the cell to ground. Due to the spin-dependent tunnel effect, the electrical resistance of the cell changes due to the change of relative orientation of the moments in the two plates. By measuring the junction resistance the magnetic state of the free layer can be determined. One may for the sake of consistency in discussions below define the two plates having the moment parallel configuration to mean “0”, while if the two plates are of anti-parallel alignment to mean “1”; the resistance will be higher and this corresponds to “1”. The same concept of using magnetic orientation to store digital information is the basis for today's magnetic hard-disk drive technology where the bits are stored by the magnetic moment orientations on the medium-plate, and a read/write head flies above the plate to access the bit state (i.e., read and write).